1. Field of the Invention
The invention relates to an improved process for separating phenol from a mixture containing hydroxyacetone, cumene, water and phenol. The invention also includes an apparatus for separating phenol from cleavage product mixtures and a composition containing cumene and xcex1-methylstyrene.
The process for the acid-catalyzed cleavage of cumene hydroperoxide into phenol and acetone has been of particular industrial importance for a long time. In the preparation of phenol from cumene by the Hock process, cumene is oxidized to cumene hydroperoxide (CHP) in a first reaction step, known as oxidation, and the CHP is subsequently concentrated to from 65 to 90% by weight in a vacuum distillation step, known as concentration. In a second reaction step, known as cleavage, the CHP is cleaved into phenol and acetone by action of an acid, usually sulfuric acid. In addition to phenol and acetone, the cleavage product further comprises additional compounds which may be formed in the reaction steps preceding the cleavage and which are only partly transformed, if at all, in the cleavage. The most important compounds present in the cleavage product in addition to phenol and acetone are, in particular, xcex1-methylstyrene (AMS), cumene and acetophenone. In addition, small amounts of dimethyl phenyl carbinol (DMPC) formed in the oxidation can be present in the cleavage product. Further impurities include compounds such as methylbenzofuran (MBF), hydroxyacetone, mesityl oxide (MO) and carbonyl compounds such as acetaldehyde and 2-phenylpropionaldehyde. After neutralization of the cleavage product and optional removal of an aqueous phase, the cleavage product is worked up by distillation.
2. Description of the Related Art
Various processes for working up the cleavage product by distillation are known (Ullmann""s Encyclopedia of Industrial Chemistry, 5th completely revised edition, Vol. A19, 1991, VCH Verlagsgesellschaft mbH, Weinheim; incorporated herein by reference). These processes involve initial neutralization of the cleavage product using aqueous sodium hydroxide, amines or aqueous phenoxide solution. After phase separation, the organic part of the neutralized cleavage product is transferred to a first column in which crude acetone is distilled off from the remaining cleavage product via the top of the column. This crude acetone is usually treated with alkali in a scrubber and once again purified by distillation. However, the scrub is sometimes also carried out in the column. The bottom product from the first column is distilled in a second column from which AMS and cumene are taken off at the top and are usually passed to a hydrogenation step in which cumene is generated. AMS and cumene can also be separated off by azeotropic distillation in the presence of water. The bottom product remaining in the second column is distilled in a crude phenol column.
The crude phenol obtained by this process can be purified further by extractive distillation using water or by treatment with an acid ion exchanger and subsequent distillation. In the latter process, compounds which are difficult to separate from phenol by distillation, e.g. mesityl oxide and hydroxyacetone, are condensed to form higher-boiling compounds.
Such a process is described, for example, in U.S. Pat. No. 5,064,507, incorporated herein by reference. In this process, the cleavage product is first separated from the crude acetone in a crude acetone column. The bottom product is transferred to a cumene column in which cumene and AMS are separated from the cleavage product. The column is, however, operated so that a certain proportion of AMS remains in the bottom product since it is required as a reactant or solvent in the further processing of the phenol to remove MBF and other impurities. This bottom product is reacted with an amine, preferably hexamethylenediamine, in a reactor having plug flow characteristics in order to convert carbonyl compounds, e.g. acetol (hydroxyacetone) or MO, into higher-boiling compounds. The product which has been treated in this way is worked up further by distillation. However, it passes through a further four columns and two reaction zones before the purified end product phenol is obtained. The outlay for apparatus required for the removal of the hydroxyacetone from the phenol is relatively high in this process.
U.S. Pat. No. 3,322,651, incorporated herein by reference, also describes the use of nitrogen compounds, in particular amines, for purifying phenol obtained in the cleavage of CHP. However, the amines have to be removed again from the products.
GB 1 021 759, incorporated herein by reference, describes the work-up of a cleavage product mixture obtained from the acid-catalyzed cleavage of CHP and from which the catalyst has been removed by neutralization and scrubbing. To be able to obtain a phenol having a low hydroxyacetone content, the separation is carried out by feeding the cleavage product mixture into a distillation column at a side inlet and fractionating this mixture in one separation step to give a top fraction comprising water, acetone, hydroxyacetone and cumene and a bottom fraction comprising phenol having a hydroxyacetone content of less than 100 ppm. Depending on the composition of the cleavage product mixture, cumene was added in an amount so that the ratio of cumene to phenol in the feed to the column was at least 0.28 part by weight of cumene to 1 part by weight of phenol, since the process is based on the separation of an azeotropic mixture of hydroxyacetone and cumene from the phenol. The hydroxyacetone can be removed from the phenol by this process. However, a top fraction which comprises not only water, cumene and hydroxyacetone but also acetone is obtained. Phenol contamination may also be present in the top fraction. These products likewise have to be separated from one another.
In U.S. Pat. No. 4,251,325, incorporated herein by reference, the work-up of a fraction which has been substantially freed of low boilers, water and acetone has been optimized by operating the cumene column in such a way that a mixture comprising cumene, AMS and hydroxyacetone is taken off at the top, with this mixture being separated virtually completely from the crude phenol remaining in the bottoms and thus not having to be removed in a costly fashion during the work-up of the phenol. This process gives phenol containing less than 30 ppm of hydroxyacetone. A disadvantage of this process is the fact that the input mixture has to be substantially free of water, which is why an acetone fraction comprising low boilers and also the major part of the water present in the cleavage product mixture has to be separated from the cleavage product mixture in the preceding separation step. The work-up of such an acetone fraction by methods of the prior art is relatively uneconomical, since a high outlay in terms of apparatus is required.